Current hydraulic systems for mobile machines are based predominantly upon valve-controlled principles. With them the various hydraulic consumers, such as drives for the working hydraulics, steering, braking etc. are controlled by means of hydraulically or electro-hydraulically driven valve arrangements. Usually one or more central pressure supplies are employed for this, frequently in the form of load-sensing pumps, which provide the flows of pressure medium, which through arrangements of one or more valves influence the desired behavior of the hydraulic consumers.
The disadvantage of this valve-controlled hydraulic system is especially the poor utilization of energy. In order to achieve the desired flows of pressure medium at the throttle edges of the valves, pressure differences are necessary, which in principle lead to high energy losses in the hydraulic valve controls. It is not possible to utilize surpluses of energy on a consumer of the system in the form of potential energy or braking energy for other consumers in the system and thereby improve the efficiency of the system, which makes the development of heat in the system worse yet again. A central pressure medium supply possesses in addition the disadvantage that where several consumers must be operated simultaneously, the volume flows are divided, which makes precise control and operation of the individual components more difficult. Safety-relevant circuits, in which it must be ensured that individual consumers, e.g. the steering or brakes, always have sufficient pressure medium available, for example always complicated priority valve arrangements must be implemented.
Simultaneous movement of several consumers in the system leads to a different system behavior in comparison with the individual movements. All of this leads to very complex, and hence expensive and maintenance-intensive valve arrangements, for which their possibilities regarding controllability and utilization of energy are limited.
Occasionally, displacement-controlled systems are used also for rotational drives, in which an adjustable pump that is variable in its displacement volume is used for the control or regulation of the motion of the hydraulic motor(s). The consumer is hence controlled only via the volume flow provided by the pump, without the use of a control valve or similar device in the main circuit. In the transfer of this control principle to linear drives with a differential cylinder, the problem arises that the cylinder volumes on both sides of the cylinder piston are different and hence with the motion differential volume flows occur, which must be compensated for by means of various known solutions.
The previously known displacement controlled systems of this type are extremely inflexible, but possess a large number of additional components or displacement units and do not offer the range of functions and system simplicity that is necessary for use in mobile machines (e.g. DE 40 08 792 A1, DE 27 06 091 A1, CA 605 046, DT 23 49 351 and Rahmfeld and Ivantysynova 2000, Energy-saving controlled linear drive with a differential cylinder, 2. IFK, pp. 191-205, Dresden).
The object of the present invention is therefore to create a hydraulic system for linear drives with a differential cylinder, in particular for mobile machines which through the use of displacement-control of the drives which avoids the many and diverse disadvantages of the state of the art and renders possible a precise and energy-efficient control of linear drives with differential cylinders, and which is economical and simple to maintain and which can be well integrated into the total hydraulic system of such machines. This object of the invention is solved by the characteristics of claim 1.
The non-return-valves are located between the two high-pressure pipes, which lead from the pump with variable delivery volume to the differential cylinder, and the common low-pressure system. If a volume flow is produced by the variable displacement pump and hence the differential cylinder is moved, depending upon the direction of movement of the piston, positive or negative difference volume flows can flow into the low-pressure system or be sucked out of it. In the case of sucking of the volume flow out of the low pressure system, the corresponding non-return valve opens automatically. In the event of the volume flow flowing out of the low-pressure system, the appropriate non-return valve is released by the high pressure of the system.
For the implementation of a floating position, the two sides of the differential cylinder must be connected hydraulically with each other, as a result of which a free movement of the piston is rendered possible. At the same time the non-return-valves are released, so that pressure medium can flow through them in both directions independently of the pump volume flow. The differential volume flow is likewise compensated for in this case by the low-pressure system.
The use of an electronic controller for switching the non-return-valves permits the valves for example to be released on demand by the operator and hence the floating position can be implemented. In addition, it offers the advantage that such a changeover occurs only if certain pressure relationships prevail in the high pressure circuit, so that switching surges or other unwanted conditions are prevented and a supporting of the load existing on the differential cylinder is always prevented. In addition, such a controller permits further functions of such a displacement-controlled circuit, which will be described in more detail below.
Thus it can be envisioned that the control device for regulating the pumps' delivery volume is formed electronically. The delivery volume of the variable flow pumps is usually controlled electro-hydraulically. Therefore it is particularly advantageous if this controller is designed to be integrated together with the control device for the non-return-valves, so that reliable and precise control of the complete circuit behavior is possible. Thus for example, it can be prevented that the pump on the non-return-valves being released delivers a volume flow, which then would briefly be short circuited by the released non-return-valves.
A further embodiment of the hydraulic system envisions that the electronic control device for triggering the check valves possesses an electro-hydraulic 4/2-way valve. By means of such a valve, the releasing connection of the check valve can be connected alternately with one or other side of the high pressure circuit, which corresponds to a changeover between the normal differential volume compensation and the floating position of the differential cylinder. As a result, the position of the non-return-valves is adjusted in accordance with the applied load and hence the pressure relationships within the cylinder. Thus a secure facility to change the operating states is created, as a result of which the risk of pressure surges is minimized.
Alternatively, two electro-hydraulic 3/2-way valves can be employed. A particular embodiment of the hydraulic system envisages that on at least one connection of the differential cylinder a controllable shutoff valve is provided. By means of such a shutoff valve, a connection of the cylinder can be closed off leak-free, which is sensible especially for the implementation of a holding function. At the same time the cylinder is brought to a certain position by the volume flow of the pump and then the high-pressure connection of the differential cylinder is closed off, so that this remains in its position, even if the pump does not maintain the pressure. If on the second connection of the differential cylinder likewise a shutoff valve is provided, the cylinder can be isolated completely from the hydraulic circuit, as a result of which it remains in its position. Through the pump and the connected hydraulic circuit, in this condition a further differential cylinder can be operated, which is likewise isolatable from the circuit by shutoff valves. As a result of this, a further function of the machine can be implemented simply and economically, which can be operated alternatively to the other existing cylinders.
It can be advantageous that the low-pressure system is formed as an accumulator filling circuit with an accumulator-filling valve, a pump with hydraulic reservoir and a pressure-limiting valve. Such an arrangement of the low-pressure side is characterized by a particularly high energy-efficiency. The pump delivers only when the pressure in the low-pressure system falls below a set minimum pressure value. The accumulator filling circuit takes care of maintaining of a low-pressure level between adjustable limits. Such a low-pressure system can be formed centrally for the entire hydraulic system and supply all of the displacement-controlled hydraulic circuits in accordance with the invention.
A further embodiment of the hydraulic system in accordance with the invention is characterized in that the controllable shutoff valve is formed as a seat valve with 3/2-way pilot control. Furthermore it can be sensible, that the controllable shutoff valve is designed as a pneumatic continuous valve. With such a valve, the appropriate blocking function of the connection can be realized simply, without an overly jerky opening and closing of the valve occurring. In this manner undesirable pressure peaks in the system can be prevented.
It could be advantageous that further constantly controllable shutoff valves are provided for alternative and/or simultaneous control of further differential cylinders. As described above, through such valves further functions on the same high-pressure circuit can be implemented, as a result of which these operate always alternatively to each other. The shutoff valves are connected in such a way that the pump with the associated protective and equalization valves is connected to one differential cylinder or several connected together with the same function and supplies these with pressure medium.
A further embodiment of the invention envisages that on the high-pressure circuit connections for a passive oscillation damping system are provided. Such damping systems consist of a hydraulic circuit with a reservoir which reduces the vibrations in the implement that occur for example when running with increased load. For this, the vibration damping system is connected directly to at least one connection on one side of the high-pressure circuit and can be switched on and off, in order to suppress the unwanted vibrations in the desired operating conditions.
In an embodiment of the invention it is envisioned that the electronic control device, which contains the controllable valves and possibly further existing hydraulic system components with the variable displacement pump is formed as an integrated component. Such integration of the pump with a series of valves and the controller offers the advantage of an extremely compact construction, which can be sensible as these components are necessary for each hydraulic function driven by differential cylinder systems. Through this integration, the number of individual components is reduced, the complexity of the overall system is reduced, the cost of installation is lowered, and thus costs of such a system is lowered in comparison with conventional systems.
It can be advantageous for the control and regulation concepts that sensors for recording the system state, in particular, the differential cylinder position and the hydraulic pressures, are provided. Furthermore it can be sensible that an electronic control device for regulating the controllable system components depending upon the measured system state and user settings is envisioned. By measuring the system state and processing the data thus obtained in a control device, the linear cylinder can be operated in a closed control circuit, which significantly improves the precision of positioning and the stability of the system.
The drive system in accordance with the invention can also be controlled, i.e. operated, in an open loop.
Furthermore, the invention is orientated towards a mobile machine with at least one hydraulic system, as described in the foregoing. In an embodiment of such a machine, several high-pressure circuits with a common low-pressure circuit is envisioned. This has, as already explained, the advantage of additional cost savings, as a single low-pressure circuit with a pump and the additional components for supplying all of the hydraulic systems in accordance with the invention suffices.
The invention is explained in more detail below with references to specific figures also provided.